1. Field of the Invention
The present invention relates to dielectric ceramic compositions for use as resonators employed in a microwave frequency band of several gigahertz.
2. Description of the Prior Art
In recent years, attempts have been made to use a dielectric material for a resonator or a filter used in satellite communication, broadcasting and microwave remote communication using a microwave a frequency band of several gigahertz. A transmitter-receiver such as a microwave remote recognition system is also sought.
Examples of this type of dielectric ceramic material conventionally used include a composition of a BaO-TiO.sub.2 -Nd.sub.2 O.sub.3 -Bi.sub.2 O.sub.3 system which is proposed in, for example, Japanese Patent Laid-Open Gazette No. 8806/1986. In this conventional dielectric ceramic composition, its dielectric constant .epsilon. is as high as 70 to 90. In addition, the temperature coefficient of resonance frequency .tau.f of the dielectric ceramic composition is also high, i.e., +10 to about +20 PPM/.degree.C., so that sufficient properties cannot be obtained.
Meanwhile, when a dielectric resonator is constructed, the higher the dielectric constant .epsilon. of a material used, the smaller the size the resonator can be. Accordingly, a material having a higher dielectric constant .epsilon. is desired.
Examples of a material having a high dielectric constant .epsilon. include SrTiO.sub.3 and CaTiO.sub.3. However, these cannot be used because the dielectric constant .epsilon. is very high, i.e., 300 and 180, while their temperature coefficient of resonance frequency .tau.f is very high, i.e., +1700 PPM/.degree.C. and +800 PPM/.degree.C.
Examples of a method of reducing the temperature coefficient of resonance frequency .tau.f of such a dielectric ceramic composition include a method of combining a material having a dielectric constant .epsilon. which is as high as possible and a temperature coefficient of resonance frequency .tau.f which takes a minus value with the dielectric ceramic composition. According to this method, a ceramic composition having a high dielectric constant .epsilon. and having a low temperature coefficient of resonance frequency .tau.f is obtained by a suitable combination.
In general, however, as the dielectric constant .epsilon. becomes higher, the temperature coefficient of resonance frequency .tau.f becomes larger on the plus side. A material having a high dielectric constant .epsilon. and a temperature coefficient of resonance frequency .tau.f which is large on the minus side has not been known.